You’ll need 2 Ones or a One and a One SL for the rear surround speakers. As I understand it, it will then be Atmos but without height effect sounds from behind. 

Hello! No it’s not for the rear surrounds. I already have two play 1 for that. I’ts when I pair my arc with the one which is not in the same room...

No. It will revert to Lossless on the Arc. I just tried it, grouping with another room while playing an Atmos track on the Arc. Once you ungroup and restart the track on the Arc, it becomes Dolby Atmos again.

EDIT: The above referred to music. If playing a TV show in Atmos, then YES, the show remains in Atmos on the Arc when you group with another room.

Thank you very much!

Don’t miss my edit/2nd post too!! :)